The present invention relates generally to a cellular mobile communication system and a radio communication apparatus, and more particularly to a handoff control method and apparatus in a mobile communication system which enable a mobile station to simultaneously use a plurality of radio channels to communicate with base stations.
In a CDMA (code division multiple access) mobile communication system, a handoff scheme called a xe2x80x9csoft handoffxe2x80x9d has been brought into practice. The soft handoff is described in U.S. Pat. No. 5,267,261.
The soft handoff is a handoff method which is characterized by uninterrupted communications and a cell diversity effect on the boundary between two cells. The soft handoff is implemented by utilizing a property of the CDMA scheme which permits simultaneous reception of a plurality of radio channels. More specifically, on uplink channels, a signal transmitted by a mobile station is received by a plurality of radio base stations, and the received signal is selected or combined in base station controller associated with the respective radio base stations to realize the above-mentioned property. On downlink channels, on the other hand, a plurality of radio base stations simultaneously transmit signals having the same information, which have been applied with different spreading, to a single mobile station, and the mobile station simultaneously receives the respective signals, despreads them to extract the same information, and then selects or combines the extracted information to realize the above-mentioned property.
However, during a soft handoff, since a plurality of radio base stations are each transmitting to a single mobile station an amount of information identical to that transmitted when no handoff is made, the single mobile station uses a number of downlink channels during a handoff several times larger as compared with that required when no handoff is made. In other words, when the soft handoff is applied, the number of downlink radio channels equal to the number of mobile stations involved in the soft handoff are used at minimum, thereby causing an increase in the amount of co-channel interference of down link channels and a decrease in the capacity of subscribers in the entire system. In addition, the soft handoff might cause each radio base station to run short of radio resources, hereby increasing the possibility of failure in setting new call.
As mentioned above, the conventional soft handoff implies a problem that the system capacity is reduced due to an increased amount of used radio resources in radio base stations and an increased transmission power. In particularly, an asymmetric high-speed circuit data communication, which is expected to experience a rapid increase in demands in near future, uses a large amount of radio resources in the downlink channels and transmission power, so that the high-speed circuit data communication, if the conventional soft handoff is applied thereto, would significantly affect the downlink channels of other users. However, if a hard handoff were applied to the channel switching high-speed data communication, this would introduce a problem of a large amount of dropped data due to interrupted communications.
The present invention has been made in view of the problem mentioned above, and its object is to provide handoff means which maintains a consistent amount of used radio resources in a radio base station regardless of whether a handoff is being made or not, while preserving the advantages of the conventional soft handoff, i.e., uninterrupted communications and cell diversity effect.
To achieve the above object, the present invention provides a handoff method, wherein a radio base station comprises means for performing error correction encoding and interleaving on transmission information, dividing the processed transmission information into a plurality of blocks, and transmitting each of the blocks through a plurality of downlink radio channels, a mobile station comprises means for simultaneously receiving the plurality of downlink radio channels transmitted from the plurality of radio base stations to the mobile station, and means for demodulating each of received signals and rearranging and decoding the demodulated signals to extract original information, wherein as the mobile station hands off from a first radio base station currently connected thereto to a second radio base station, a plurality of downlink radio channels are handed off in order at least on a one-by-one basis, and while the plurality of downlink radio channels are being handed off at least on a one-by-one basis, the remaining downlink radio channels continue communications.
In another aspect, the present invention provides a radio communication system having a mobile station and a plurality of radio base stations, wherein each of the radio base stations comprises an encoder for encoding transmission information directed to the mobile station, a divider for dividing the encoded transmission information into a plurality of partial channels, and a transmitter for transmitting the partial channels, the mobile station comprises a receiver for receiving the plurality of partial channels, a combiner for combining the plurality of received partial channels, and decoder for decoding the combined transmission information, and the mobile station hands off the plurality of partial channels at least on a one-by-one basis, when handing off from a first radio base station to a second radio base station.
In the radio base station, the encoder preferably encodes transmission information using a convolutional code.
When the plurality of partial channels are handed off to the second radio base station, the mobile station may continuously communicate a radio channel, which has not been handed off, through the first radio base station.
The radio base station preferably includes a switch for setting whether or not transmission is possible for each of the plurality of partial channels.
In a further aspect, the present invention provides a radio communication system having a mobile station and a plurality of radio base stations, wherein each of the radio base stations comprises an encoder for encoding transmission information directed to the mobile station, a divider for dividing the encoded transmission information into a plurality of partial channels, an orthogonal encoder for orthogonally encoding the plurality of partial channels respectively with orthogonal codes which are orthogonal to each other, a spreader for spreading the orthogonally encoded partial channels with the same spread code, and a transmitter for transmitting the spread partial channels, the mobile station comprises a receiver for receiving the plurality of partial channels, a despreader for despreading the plurality of received partial channels, a combiner for combining the despread partial channels, and decoder for decoding the combined transmission information, and the mobile station hands off the plurality of partial channels at least on a one-by-one basis, when handing off from a first radio base station to a second radio base station.
Also, when the plurality of partial channels are handed off to the second radio base station, the mobile station may continuously communicate a radio channel, which has not been handed off, through the first radio base station.
The radio base station preferably includes selecting means for setting whether or not transmission is possible for each of the plurality of partial channels. The selecting means may be a hardware switch or a software switch.
Further, in a radio communication system having a mobile station and a plurality of radio base stations, the radio base stations each comprises an encoder for convolutional encoding transmission information directed to the mobile station, a divider for dividing the convolutional encoded transmission information into a plurality of partial channels, and a transmitter for transmitting the partial channels, wherein the radio base station hands off the plurality of partial channels at least on a one-by-one basis when the mobile station in communication with the radio base station initiates a handoff.
Further, in a radio communication system having a mobile station and a plurality of radio base stations, the radio base stations each comprises an encoder for encoding transmission information directed to the mobile station, a divider for dividing the encoded transmission information into a plurality of partial channels, an orthogonal encoder for orthogonally encoding the plurality of partial channels respectively with orthogonal codes which are orthogonal to each other, a spreader for spreading the orthogonally encoded partial channels with the same spread code, and a transmitter for transmitting the spread partial channels, wherein the radio base station hands off the plurality of partial channels at least on a one-by-one basis when the mobile station in communication with the radio base station initiates a handoff.
Further, in a radio communication system having a mobile station and a plurality of radio base stations, the mobile station comprises a receiver for receiving a plurality of partial channels transmitted by the radio base stations, a combiner for combining the plurality of received partial channels, and a decoder for decoding the combined transmission information, wherein the mobile station hands off the plurality of partial channels at least on a one-by-one basis when handing off with the plurality of radio base stations.
Further, in a radio communication system having a mobile station and a plurality of radio base stations, the mobile station comprises a receiver for receiving a plurality of partial channels transmitted by the radio base stations, a despreader for despreading the plurality of received partial channels, a combiner for combining the despread partial channels, and a decoder for decoding the combined transmission information, wherein the mobile station hands off the plurality of partial channels at least on a one-by-one basis when handing off with the plurality of radio base stations.
In a further aspect, the present invention provides a radio communication apparatus for communicating with a plurality of base stations using a CDMA scheme, which comprises a signal search element for detecting spread code offsets of respective signals transmitted by the plurality of base stations, demodulation elements each for despreading one of the signals using an associated spread code having an offset detected by the signal search element to demodulate the one signal, a first symbol combiner for RAKE combining signals transmitted from the same base station within the despread and demodulated signals, orthogonal code correlators each for despreading a signal RAKE combined by the first symbol combiner with an appropriate orthogonal code to extract a plurality of blocks, a second symbol combiner for RAKE combining each of the blocks extracted from the orthogonal code correlators, a multiplexer circuit for multiplexing outputs of the symbol combiner, and a reception system for rearranging and decoding an output of the multiplexer circuit to extract original information.
In a further aspect, the present invention provides a handoff method for a radio communication system comprising a mobile station, a plurality of radio base stations, and a controller for controlling a handoff of the mobile station between the radio base stations, wherein the controller transmits a first partial channel transmission start instruction signal for transmitting a first partial channel to a handoff destined radio base station within the plurality of radio base stations, and the handoff destined radio base station transmits the first partial channel based on the first partial channel transmission start instruction signal, the controller transmits a first partial channel transmission stop instruction signal for stopping the transmission of the first partial channel to a handoff initiated radio base station when the mobile station has connected the first partial channel to the handoff destined radio base station, the handoff initiated radio base station, upon receipt of the first partial channel transmission stop instruction signal, stops transmitting the first partial channel, the controller, upon starting/completing a handoff of the first partial channel, transmits a second partial channel transmission start instruction signal to the handoff destined radio base station within the plurality of radio base stations, and the handoff destined radio base station transmits the second partial channel based on the second partial channel transmission start instruction signal, the controller transmits a second partial channel transmission stop instruction signal for stop transmitting the second partial channel, which has been connected, to the handoff initiated radio base station when the mobile station has connected the second partial channel to the handoff initiated radio base station, and the handoff initiated radio base station, upon receipt of the second partial channel transmission stop instruction signal, stops transmitting the second partial channel.
In a further aspect, the present invention provides a handoff method for a radio communication system comprising a mobile station, a plurality of radio base stations, and a controller for controlling a handoff of the mobile station between the radio base stations, wherein the controller transmits a first partial channel transmission start instruction signal for transmitting a first partial channel to a handoff destined radio base station within the plurality of radio base stations, and the handoff destined radio base station, based on the first partial channel transmission start instruction signal, encodes transmission information directed to the mobile station, divides the encoded transmission information into a plurality of partial channels, orthogonally encodes the plurality of partial channels respectively with orthogonal codes which are orthogonal to each other, spreads the orthogonally encoded partial channels with a spread code, and transmits a first partial channel within the spread partial channels, the controller transmits a first partial channel transmission stop instruction signal for stopping the transmission of the first partial channel, which has been connected, to a handoff initiated radio base station when the mobile station has connected the first partial channel to the handoff destined radio base station, the handoff initiated radio base station, upon receipt of the first partial channel transmission stop instruction signal, stops transmitting the first partial channel, the controller transmits a second partial channel transmission start instruction signal to the handoff destined radio base station within the plurality of radio base stations, the handoff destined radio base station, based on the second partial channel transmission start instruction signal, encodes transmission information directed to the mobile station, divides the encoded transmission information into a plurality of partial channels, orthogonally encodes the plurality of partial channels respectively with orthogonal codes, which are orthogonal to each other, spreads the orthogonally encoded partial channels with a spread code, and transmits a second partial channel within the spread partial channels, the controller transmits a second partial channel transmission stop instruction signal for stop transmitting the second partial channel, which has been connected, to the handoff initiated radio base station when the mobile station has connected the second partial channel to the handoff destined radio base station, and the handoff initiated radio base station, upon receipt of the transmission stop instruction signal, stops transmitting the second partial channel.
In a further aspect, the present invention provides a handoff method for a radio communication system comprising a mobile station, a plurality of radio base stations, and a controller for controlling a handoff of the mobile station between the radio base stations, wherein a first radio base station within the radio base stations divides information to be transmitted to the mobile station into a plurality of blocks, selects a block to be transmitted to the mobile station within the plurality of blocks based on an instruction signal from the controller, orthogonally encodes the selected block, spreads the orthogonally encoded block with a spread code, and transmits the spread block to the mobile station, a second radio base station within the radio base stations divides information to be transmitted to the mobile station into a plurality of blocks, selects a block to be transmitted to the mobile station within the plurality of blocks based on an instruction signal from the controller, orthogonally encodes the selected block, spreads the orthogonally encoded block with a spread code, and transmits the spread block to the mobile station, and the mobile station receives blocks transmitted from the first and second radio base stations, and diversity combines the orthogonally encoded blocks with the same orthogonal code.
As described above, the present invention can realize a handoff without causing a change in the amount of used radio resources available to a radio base station when a channel is switched, and without suffering from disruption. It is therefore possible to save radio resources and transmission power of the radio base station as well as to increase the capacity of subscribers for the entire system, as compared with a conventional soft handoff. This effect is significant particularly when the present invention is applied to high speed data communications which use a large amount of radio resources and a high transmission power. In addition, the present invention can be readily applied to a conventional CDMA radio communication system.